The present invention relates to an image forming apparatus employing an electrostatic transfer process such as an electrophotographic copying machine, an electrostatic recording apparatus.
For charging an image forming object such as a photoreceptor drum or the like, there has hitherto been used generally a corona charger wherein high voltage is impressed on a discharge wire and thereby a strong electric field is generated around the discharge wire for gaseous discharge. The image forming object is charged when electric charge ions generated in the course of gaseous discharge are adsorbed on the image forming object.
A corona charger used in the conventional image forming apparatus mentioned above has an advantage that an image forming object is not damaged in the course of charging thereon because the charger does not come into mechanical contact with the image forming object. The corona charger, however, has a disadvantage, due to high voltage used therein, that there is a risk of an electric shock or electric leakage and also ozone generated in the course of gaseous discharge is harmful to the human body and the ozone shortens a life of the image forming object. Further, charging voltage by means of a corona charger is sharply influenced by temperature and humidity to be unstable, and noise is caused by high voltage in the corona charger, which is a serious disadvantage on the occasion where an electrophotographic image forming apparatus is utilized as a terminal equipment for communication or an information processing apparatus.
These many disadvantages of a corona charger are caused by gaseous discharge necessary for charging.
Therefore, there are disclosed in Japanese Patent Publication Open to Public Inspection Nos. 133569/1984, 21873/1992 and 116674/1992 (hereinafter referred to as Japanese Patent O.P.I. Publication) the charging devices wherein magnetic particles are adsorbed on a cylindrical conveying carrier which is a charging roller holding therein magnetic objects for forming a magnetic brush, and the magnetic brush rubs the surface of the image forming object for charging it, as a charging device capable of charging the image forming object without conducting high voltage gaseous discharge carried out in a corona discharge and without giving any mechanical damages on the image forming object.
However, even the charging devices disclosed in the aforementioned Japanese Patent O.P.I. Publications have had a problem that an image forming object can not be charged uniformly, completely and stably. Namely, the problem is that when an amount of magnetic particles on the surface of the charging roller is not kept appropriately in the charging area, uneven charging or local excessive charging takes place, or dielectric breakdown of the image forming object caused by bias voltage takes place.
FIG. 5 represents an enlarged schematic view showing a charging section of a conventional charging device wherein magnetic particles 21 on the surface of charging roller 22 form magnetic brush 21A protruded along a line of magnetic force in the form of a chain and bias voltage is impressed by bias power source 24 between image forming object 10 and charging roller 22 for charging through magnetic brush 21A, because a magnetic pole of magnet body 23 affixed inside charging roller 22 that represents the conveying carrier is located in the vicinity of a charging area. In this case, a tip of magnetic brush 21A comes directly into contact with the image forming object 10, resulting in occurrence of local excessive charging, dielectric breakdown of the image forming object 10 and uneven charging thereon, which is a problem. In addition, when the image forming object 10 is subjected to dielectric breakdown in a form of a dot, potential of the total magnetic brushes 21A is lowered through the dot, and there is represented an improper charging area by an area whose width is broader than other areas in a contact strip zone in the direction perpendicular to that of rotation of the image forming object 10 where the magnetic brush 21A is in contact with the image forming object 10.
Further, as described above, there are disclosed in Japanese Patent Publication Open to Public Inspection Nos. 133569/1984 and 187267/1988 the other charging devices wherein magnetic particles are adsorbed on a cylinder holding therein magnetic objects for forming a magnetic brush, and the magnetic brush rubs the surface of the image forming object for charging it, as a charging device capable of charging the image forming object without conducting gaseous discharge carried out in a corona discharge and without giving any mechanical damages on the image forming object.
In the case of charging by means of a magnetic brush, the magnetic brush is caused to make sufficient contact with an image forming object and is caused to rub, and electrons are injected under the condition of relatively high alternating electric field, both for charging the image forming object sufficiently. Due to the high alternating electric field, uneven charging is sometimes caused on the image forming object, or magnetic particles stick to the image forming object. When magnetic particles stick to the image forming object in the course of charging by means of a magnetic brush, uneven toner images are caused in the succeeding process such as imagewise exposure or developing, image quality is disturbed, an image forming object is damaged or an image forming apparatus is deteriorated in terms of its efficiency.
In the technologies disclosed in the aforementioned Japanese Patent O.P.I. Publications, therefore, magnetic particles stuck to an image forming object after charging by means of a magnetic brush are scraped off by a blade brought into contact with the image forming object for scraping after charging, and are collected to the magnetic brush side.
In the method to scrape off magnetic particles sticking to an image forming object by means of a blade, however, there is a fear that damage on the image forming object is rather increased, and in addition, the blade is damaged soon by magnetic particles which are relatively hard and large, which is different from the case of blade-cleaning of toner, and a function of the blade can not be demonstrated sufficiently.